Implantable cardiac devices are well known in the art. They may take the form of implantable defibrillators or cardioverters, which treat accelerated rhythms of the heart such as fibrillation or implantable pacemakers, which maintain the heart rate within a prescribed limit, such as, for example, to treat a bradycardia. Implantable cardiac devices are also known which incorporate both a pacemaker and a defibrillator.
Pacemakers deliver pacing pulses to the heart to cause the stimulated heart chamber to contract when the patient's own intrinsic rhythm fails. To this end, pacemakers include sensing circuits that sense cardiac activity for the detection of intrinsic cardiac events such as intrinsic atrial events (P waves) and intrinsic ventricular events (R waves). By monitoring such P waves and/or R waves, the pacemaker circuits are able to determine the intrinsic rhythm of the heart and provide stimulation pacing pulses that force atrial and/or ventricular depolarizations at appropriate times in the cardiac cycle when required to help stabilize the electrical rhythm of the heart.
Implantable cardiac defibrillators (ICD's), encapsulated in a conductive housing or enclosure, are generally implanted in the left pectoral region of a patient and electrically connected to the heart with one or more electrode carrying leads. One lead includes at least one set of electrodes positioned in the right ventricle. An arrhythmia detector detects ventricular arrhythmias, such as ventricular fibrillation. When such an arrhythmia is detected, a pulse generator delivers a defibrillation output pulse from the defibrillation electrode in the right ventricle to the conductive housing to terminate the arrhythmia. Alternatively, such arrhythmia terminating systems may further include another defibrillation electrode arranged to be positioned in the right atrium and electrically connected to the right ventricular defibrillation electrode. In this arrangement, the defibrillation output pulse is delivered from the parallel connected right ventricular and right atrial electrodes to the conductive housing.
When ICDs are initially implanted their functionality is checked by inducing fibrillation in the heart of the patient and allowing the ICD to correct the condition. The process of inducing fibrillation applies a sudden direct current (DC) having a squared waveform and fixed duration to the heart. This sudden application of DC results in pain and discomfort for the patient, as the patient may be conscious although medicated. Inducement of fibrillation may also be used periodically after implant to recalibrate or check the function of the ICD. Fibrillation may be induced at the time that medication is changed to ensure that device is capable of defibrillating the heart when the patient is on the new medication.